Generally, when a voltage between both ends of a cell (a battery cell) exceeds a predetermined value, there is a risk of explosion and when the voltage drops below a predetermined value, damage may be caused in the battery cell. Specifically, since a hybrid electric vehicle or a notebook computer requires a relatively large quantity of power supply, when the power is supplied using a battery cell, a battery module (battery pack) in which battery cells are connected in series) needs to be used therefor. However, when such a battery module is used, an imbalance of voltage may be caused due to a performance deviation of individual battery cells.
Further, when the battery module is charged, if one battery cell in the battery module reaches an upper limit voltage earlier than other battery cells, the battery module is not charged any more. Therefore, the charging is completed in a state in which other battery cells are not sufficiently charged. In this case, a charging capacity of the battery module does not reach a rating charging capacity.
In the meantime, when the battery module is discharged, if one battery cell in the battery module reaches a lower limit voltage earlier than other battery cells, the battery module cannot be used any more. Therefore, a usage time of the battery module is correspondingly shortened.
Further, a voltage required in a load of a motor of a vehicle which uses a lithium ion battery pack is high (upto 400 V), so that a plurality of battery cells (90 to 100) is connected in series to obtain high voltage. In this case, since there is a difference in production deviations or operation temperature deviations of the plurality of battery cells, an imbalance of the voltage between cells may be caused and thus the available capacity and power may be reduced and aging of the battery cell may be accelerated.